One of the electrochemical methods for oxygen determination is the amperometric method. This method is quite rapid, simple in operation and is especially suited for determining either gaseous or dissolved oxygen in liquids. Sensors utilized in the amperometric method of oxygen detection are well known in the art and generally comprise a hollow body defining therein a reservoir for electrolyte and the body is provided with an opening for communication between the exterior and the reservoir. An anode and cathode are disposed within the body for contact with the electrolyte and a thin polymeric membrane, which is impermeable by the electrolyte but which is permeable by oxygen, seals the opening of the body. Means are provided for electrically connecting the electrodes to an electrical potential and to current measuring means. A potential is applied between the anode and the cathode and as sample fluid is brought into contact with the membrane sealing the sensor body, oxygen diffuses through the membrane to contact the cathode in the presence of electrolyte. A current flow results which is linear with the partial pressure of oxygen being sampled. The current is measured and correlated to the amount of oxygen in the sample.
Amperometric oxygen sensors can be said to generally be one of two types. With the first type of sensor, oxygen is reduced as it contacts the cathode thus causing a current flow between anode and cathode. This type of sensor requires a constant O.sub.2 flux through the membrane to avoid oxygen depletion within the sensor since O.sub.2 is reduced to the hydroxyl group at the cathode and is not regenerated at the anode. This type of sensor is very sensitive to the flow rate of the test fluid and normally requires some means for maintaining a flow of fluid to the sensor. Current flow is directly related to the partial pressure of the oxygen in the test sample. Another type of O.sub.2 sensor is the equilibrium type sensor which employs sufficient potential to cause continuous reduction of oxygen from the electrolyte at the cathode and the formation of oxygen at the anode. Once stabilized in operation, an equilibrium condition is set up within the sensor which is disturbed only when the partial pressure of O.sub.2 at the exterior surface of the membrane changes, thus disturbing the balance of the equilibrium. Disturbance of equilibrium creates a current flow which is also a direct measure of the partial pressure of oxygen in the sample. Although good results are achieved with both types of sensors, the equilibrium type sensor is preferred because it is independent of flow of oxygen past the membrane surface and does not require control of the sample flow rate or stirring of the sample fluid in the immediate vicinity of the sensor as is normally required with the first type of sensor as described above.
In the equilibrium mode of operation, sensor operation is maximized when the electrodes are concentrically disposed in the sensor body with working surfaces adjacent the inner surface of the membrane. With concentrically disposed electrodes, it is necessary that the ratio of anode surface area to cathode surface area be at least on the order of 85:1. Should the ratio of the anode surface to the cathode surface area be less than about 85:1, the sensor takes an unduly long period of time, on the order of several hours, to stabilize after any stepchange in O.sub.2 level. Also, long term stability of the sensor is poor even at constant O.sub.2 levels with a resulting continuous loss of sensitivity. It has also been found that such sensors exhibit loss of activity over a period of time.